At 17th level, the Death Domain cleric's Improved Reaper feature (DMG p. 96-97) lets them target two creatures (within 5 feet of each other) instead of one with necromancy spells, given they are within 5 feet of one another.
If I cast Inflict Wounds using Improved Reaper on two targets next to each other, can one use of the Touch of Death Channel Divinity option deal extra damage to both?
The Channel Divinity: Touch of Death feature states (DMG, p. 97; emphasis mine):
Starting at 2nd level, the cleric can use Channel Divinity to destroy another creature's life force by touch.
When the cleric hits a creature with a melee attack, the cleric can use Channel Divinity to deal extra necrotic damage to the target. The damage equals 5 + twice his or her cleric level.
Note that this feature consistently uses the singular; it targets only a single creature. This feature can be used when you hit a creature, so if you wanted it to apply to both creatures you would have to use two uses of Channel Divinity.
Something to keep in mind is how Improved Reaper works, it is quite similar to the Sorcerer's Twinned Spell Metamagic which is discussed in the following Q/A:
To summarize the answers there: when using this feature, if your spell involves an attack roll, you will make separate attack and damage rolls for each target.
